Since the 1940's optical devices in the form of intraocular lens (IOL) implants have been utilized as replacements for diseased or damaged natural ocular lenses. In most cases, an intraocular lens is implanted within an eye at the time of surgically removing the diseased or damaged natural lens, such as for example, in the case of cataracts. For decades, the preferred material for fabricating such intraocular lens implants was poly(methyl methacrylate), which is a rigid, glassy polymer.
Softer, more flexible IOL implants have gained in popularity in more recent years due to their ability to be compressed, folded, rolled or otherwise deformed. Such softer IOL implants may be deformed prior to insertion thereof through an incision in the cornea of an eye. Following insertion of the IOL in an eye, the IOL returns to its original pre-deformed shape due to the memory characteristics of the soft material. Softer, more flexible IOL implants as just described may be implanted into an eye through an incision that is much smaller, i.e., less than 4.0 mm, than that necessary for more rigid IOLs, i.e., 5.5 to 7.0 mm. A larger incision is necessary for more rigid IOL implants because the lens must be inserted through an incision in the cornea slightly larger than the diameter of the inflexible IOL optic portion. Accordingly, more rigid IOL implants have become less popular in the market since larger incisions have been found to be associated with an increased incidence of postoperative complications, such as induced astigmatism.
With recent advances in small-incision cataract surgery, increased emphasis has been placed on developing soft, foldable materials suitable for use in artificial IOL implants. One such suitable class of soft, foldable materials is silicone elastomers fabricated through the polymerization of divinyl-end capped poly(dialkyl)-co-(diaromatic substituted) siloxane with polysiloxanes having multiple hydrosilane groups. This silicone elastomer producing polymerization reaction is achieved under thermal conditions using a platinum catalyst. A component added to the described siloxane and polysiloxanes prior to initiation of the polymerization reaction, is a reinforcing agent to enhance the mechanical properties of the silicone elastomer end product so fabricated. Examples of suitable reinforcing agents include silica filler or an organosilicon reinforcing resin such as siloxane-based resin with at least one vinyl functional group.
The prepolymer, divinyl-end capped poly (dialkyl)-co-(diaromatic substituted) siloxane used in the polymerization reaction described above, is prepared by reacting a 1,3-bisvinyl tetraalkyldisiloxane, a mixture of octamethylcyclo-tetrasiloxane and an all aromatic group-containing cyclosiloxane, especially octaphenylcyclo-tetrasiloxane. Using an amine or a potassium silanoate as a catalyst, the reaction used to prepare the noted prepolymer is carried out at 40-100° C. in neat or in an organic solvent. This polymerization reaction only reaches an equilibrium with some cyclics, either those of the original components or those regenerated from the growing polymer, which then remain as side products. The resulting product was purified using a high temperature, high vacuum, thin film evaporator to remove solvent and volatile cyclics. Because of the poor solubility of the aromatic cyclics, incorporating a quantitative amount of the all-aromatic cyclics into the growing polymer molecule proved difficult. Likewise, due to high melting points (octaphenylcyclosiloxane has melting point of 197-199° C.; boiling point of 332° C./1 mmHg), the aromatic cyclics have no vapor pressures and can not be removed effectively using a thin film evaporator even at extremely high temperature (200° C.). As a result, in most cases, the aromatic cyclics, even though soluble in the compositions before curing, remain as contaminants in the final silicone elastomer product. The presence of aromatic cyclics as contaminants in the final silicone elastomer product creates the potential for defects and possible failures in products produced therefrom.
Because of the noted shortcomings in the quality of divinyl-end capped poly (dialkyl)-co-(diaromatic substituted) siloxane prepolymer using the described known process, there is a need to have an improved process for synthesizing the divinyl-end capped poly (dialkyl)-co-(diaromatic substituted) siloxane prepolymer or replacing the diaromatic substituted moiety in the prepolymer structure while maintaining high refractive index nature of this type of prepolymer.